Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. Though significant advances in high temperature capabilities have been achieved through formulation of iron, nickel and cobalt-base superalloys, alternative materials have been investigated. CMC materials are a notable example because their high temperature capabilities can significantly reduce cooling air requirements. CMC materials generally comprise a ceramic fiber reinforcement material embedded in a ceramic matrix material. The reinforcement material may be discontinuous short fibers dispersed in the matrix material or continuous fibers or fiber bundles oriented within the matrix material, and serves as the load-bearing constituent of the CMC. In turn, the ceramic matrix protects the reinforcement material, maintains the orientation of its fibers, and serves to dissipate loads to the reinforcement material. Individual fibers (filaments) are often coated with a release agent, such as boron nitride (BN), to form a de-bond layer that allows for limited and controlled slip between the fibers and the ceramic matrix material.
Continuous fiber reinforced ceramic composites (CFCC) are a type of CMC that offers light weight, high strength, and high stiffness for a variety of high temperature load-bearing applications, including shrouds, combustor liners, vanes, blades, and other high-temperature components of gas turbine engines. A CFCC material is generally characterized by continuous fibers (filaments) that may be arranged to form a unidirectional array of fibers, or bundled in tows that are arranged to form a unidirectional array of tows, or bundled in tows that are woven to form a two-dimensional fabric or woven or braided to form a three-dimensional fabric. For three-dimensional fabrics, sets of unidirectional tows may, for example, be interwoven transverse to each other. Of particular interest to high-temperature applications are silicon-based composites, such as silicon carbide (SiC) as the matrix and/or reinforcement material. SiC fibers have also been used as a reinforcement material for a variety of other ceramic matrix materials, including titanium carbide (TiC), silicon nitride (Si3N4), and alumina (Al2O3).
The fabrication of CMCs typically involves the use of multiple prepreg layers, each in the form of a “tape” comprising the desired ceramic fiber reinforcement material, one or more precursors of the CMC matrix material, and organic resin binders. According to conventional practice, prepreg tapes can be formed by impregnating the reinforcement material with a slurry that contains the ceramic precursor(s) and binders. Preferred materials for the precursor will depend on the particular composition desired for the ceramic matrix of the CMC component, for example, SiC powder and/or one or more carbon-containing materials that are ultimately converted to SiC upon reaction with molten Si. Other typical slurry ingredients include organic binders that promote the pliability of prepreg tapes, and solvents for the binders that promote the fluidity of the slurry to enable impregnation of the fiber reinforcement material.
After allowing the slurry to partially dry and, if appropriate, partially curing the binders (B-staging), the resulting prepreg tape is laid-up with other tapes, and then debulked and, if appropriate, cured while subjected to elevated pressures and temperatures to produce a preform. The preform is then heated (fired) in a vacuum or inert atmosphere to decompose the binders, remove solvents, and convert the precursor to the desired ceramic matrix material. Due to decomposition of the binders, the result is a porous CMC body that may undergo melt infiltration (MI) to fill the porosity and yield the CMC component. Melt-infiltration processes used to produce SiC matrices generally entail infiltrating the porous CMC body with molten silicon supplied externally. The molten silicon infiltrates into the porosity, reacts with the carbon content of the matrix to form silicon carbide, and fills the porosity to yield the desired CMC component.
CMCs and CFCCs articles produced to contain silicon carbide fibers in a silicon carbide matrix in the manner discussed above contain residual silicon metal, which is typically in a continuous phase interwoven between silicon carbide grains that have a small grain size and limited connectivity between adjacent grains.
This silicon phase softens and eventually melts at temperature exceeding about 1204° C., limiting the capability of the MI CMC. Moreover, even if the silicon phase is removed by an extraction process, the resulting silicon carbide material in the CMC is porous and not well-connected, making it a weak phase relative to the fiber. Thus, mechanical load that can be applied is limited at temperatures exceeding 1204° C. even for silicon extracted CMC, since the matrix can crack effectively transferring the load completely to the fibers. Such phenomena results in short creep rupture lives.
As such, a need exists for an improved CMC with higher temperature capabilities, along with methods of its manufacture.